Process for recovering carbon dioxide from stack gas

ABSTRACT

Disclosed is a process for recovering pure CO2 from stack gases produced by the combustion of carbonaceous material. The process involves contacting the stack gas with porous sodium carbonate having a high surface area, i.e., greater than 0.3 m2/gm. at a temperature of from about 40* to 75* C. Interaction between moist CO2 and sodium carbonate forms 3NaHCO3.Na2CO3 which upon heating to a temperature of from 150* to 250* C. decomposes into CO2, H2O and Na2CO3. The liberated CO2 is readily recovered and the regenerated Na2CO3 can be used to recover additional CO2 thus permitting a cyclic process.

United States Patent Frevel et al.

[ 51 May 2,1972

[54] PROCESS FOR RECOVERING CARBON DIOXIDE FROM STACK GAS [72] Inventors: Ludo K. Frevel, Midland; Leonard J.

Kressley, Saginaw, both of Mich.

[2]] Appl. No.: 8,036

[52] US Cl ..23/150, 23/25, 23/63, 23/64 [51] Int. Cl. ....Clb3l/20,B01d 53/34 [58] Field of Search ..23/2, 2.1, 4, 150, 63, 64

[56] References Cited UNITED STATES PATENTS 1,831,731 11/1931 Al ..23/ 3,511,595 5/1970 Fuchs ..23/4

Cocksedge ..23/64 Sundstrom et al. ..23/64 Primary ExaminerEarl C. Thomas AttorneyGriswold & Burdick, Jerome L. Jeffers and William R. Norris [5 7] ABSTRACT Disclosed is a process for recovering pure CO; from stack gases produced by the combustion of carbonaceous material. The process involves contacting the stack gas with porous sodium carbonate having a high surface area, i.e., greater than 0.3 m lgm. at a temperature of from about to C. 1nteraction between moist CO and sodium carbonate forms 3NaHCO -Na CO which upon heating to a temperature of from to 250 C. decomposes into CO H 0 and Na,co,. The liberated CO is readily recovered and the regenerated Na CO can be used to recover additional CO thus permitting a cyclic process.

8 Claims, No Drawings PROCESS FOR RECOVERING CARBON DIOXIDE FROM STACK GAS BACKGROUND OF THE INVENTION Stack gases resulting from the combustion of carbonaceous materials such as coke, coal, oil and natural gas serve as a major source of carbon dioxide. Normally, these fuels are burned in a manner which produces a gas containing 16 to 18 percent carbon dioxide. Other gases such as oxygen, nitrogen, nitric oxide, carbon monoxide and sulfur dioxide are normally present in significant amounts in stack gas. The CO contained in the stack gas is a useful by-product and its recovery is desirable.

One method of separating CO from such a gas mixture is to scrub the gas by passing it countercurrent to an aqueous solution of sodium carbonate. The process results in the formation of a sodium bicarbonate solution from which CO is recovered upon heating. In this method, S and other acid gases which react with the sodium carbonate solution are normally removed before the scrubbing operating.

There are several advantages to be realized by the use of solid sodium carbonate for the recovery of CO For example, the solid system can be employed in an ordinary packed column without the necessity of scrubbing apparatus. Additionally, the heat transfer problems involved in regenerating CO from a solid system are less than in a liquid system. However, the use of solid sodium carbonate granules has not been found to be an efficient method of recovering CO from stack gas because only small amounts of CO are absorbed on the surface of the granules.

It would be desirable, and it is a principal object of the present invention to provide an efficient process for the recovery of CO from stack gas which employs solid sodium carbonate as the reactive agent.

A further object is to provide such a process which provides for the recovery of CO in economical amounts.

An additional object is to provide such a process in which the sodium carbonate can be regenerated after reaction with the CO so as to be useful in a cyclic system.

SUMMARY OF THE INVENTION The invention is a process for recovering CO from the gases produced by the combustion of carbonaceous materials. The process involves contacting the stack gas with porous sodium carbonate having a BET surface area of greater than 0.3 m /gm. at a temperature of from about 40 to 75 C. The interaction between moist CO and carbonate results in the formation of 3Nal-ICO -Na CO which upon heating to a temperature of from about 150 to 250 C. dissociates the bicarbonate into CO H 0 and Na CO The liberated CO is then recovered by conventional means.

DETAILED DESCRIPTION OF THE INVENTION Sodium carbonate forms having the surface area necessary for use in the process can be prepared by grinding solid carbonate into small particles which are then compacted.

In a preferred embodiment, the porous sodium carbonate is prepared by the thermal decomposition of porous sodium bicarbonate having a BET surface area of greater than 0.3 m lgm. The bicarbonate granules preferably have BET surface areas of from 0.5 to 2.0 m /gm. to provide carbonate having similar surface areas. Carbonate granules having BET surface areas of 0.5 to 2.0 m lgm. are preferred for use in the process. Suitable porous sodium bicarbonate may be prepared by reacting ammonium bicarbonate with a soluble sodium salt in aqueous solution wherein the concentration of sodium ion is from 5 to 7 grams ions per liter in the presence of carbon dioxide in sufficient quantity to create a partial pressure of from to 40 psig while vigorously agitating the solution to form fine crystals of sodium bicarbonate. The fine crystals are separated from the reaction liquor, compacted and dried at a temperature of from 30 to 40 C. This process, which is more fully described in a copending application for a Process for the Preparation of Porous Sodium Bicarbonate executed by Ludo K. Frevel and Leonard J. Kressley at Midland, Michigan on Nov. 24, 1969 and filed in the United States Patent Office on Nov. 26, 1969 as application Ser. No. 880,424, produces hard porous sodium bicarbonate granules having a BET surface area of greater than 0.3 m /gm. The thermal decomposition of these sodium bicarbonate granules provides sodium carbonate granules suitable for use in the present process. The sodium bicarbonate is normally heated to a temperature of from to C. in order to decompose it into sodium carbonate.

The stack gas can be contacted with the sodium carbonate in a variety of ways. Conveniently, the gas is allowed to pass upwardly through a column packed with the sodium carbonate granules or with a mixture of sodium carbonate and some inert porous material. Since stack gas normally contains acid gases such as S0 and N0 the recovery of purified CO is facilitated by removing these gases before contacting the stack gas with the sodium carbonate. Additionally, the reaction of the acid gases with the carbonate is irreversible so their removal is necessary for the operation of a cyclic process. The acid gases can be removed from the stack gas by a number of methods. One such method is to pass the gas through a bed of the porous sodium bicarbonate described above. After removal of the acid gases, the remaining stack gas consists essentially of N CO 0 and small amounts of NO. Stack gas will also contain from about 3 to 25 volume percent water vapor when maintained at a temperature above its dew point. The presence of water vapor in the stack gas is necessary to provide water for the overall reaction:

while the nonreactive N 0 and NO pass through the sodium carbonate bed. Cooling the gas to a temperature of from 40 to 75 C. and preferably to between 50 and 65 C. causes it to become more saturated in water vapor and enhances the reaction. However, when cooling the gas, care should be taken not to cool it below its dew point since condensation within the carbonate bed will diminish its efficiency. In order to achieve maximum recovery of CO water should be present in amounts at least equimolar with the CO An excess of water is preferred provided the water content of the gas is not so high that the dew point is reached at the reaction temperature of 40 to 75 C.

The above reaction is reversed by heating the reaction product of sodium carbonate and CO to a temperature of from 150 to 250 C., thereby forming sodium carbonate, carbon dioxide and water vapor. Virtually 100 percent pure CO is recovered upon condensation of the water vapor and venting the CO gas into a reservoir. The regenerated sodium carbonate may then be reused for the recovery of additional CO Mixing the sodium carbonate with a porous inert material such as foam glass will add mechanical support to the system so that many cycles can be carried out before replacement of the sodium carbonate granules is necessary.

EXAMPLE I A l by 36 inch jacketed iron-pipe reactor was loaded with 255 gm. of 4-6 mesh NaI-ICO which had a BET surface area of approximately 1 m lgm. The reactor was then heated with steam in the jacket for a period of 3-4 hours to various temperatures up to C. and the liberated CO and water vapor collected by condensing the water in a small water cooled trap and venting the CO to a steel tank. During the heating cycle pressure in the reactor was held at 40-50 psig by a relief type pressure control valve. The temperature in the reactor was then lowered to between 55 and 70 C. by circulating water through the jacket.

Synthetic stack gas, made up by blending flowing streams of N 0 NO and CO in ratios encountered in stack gases formed by the combustion of solid and liquid carbonaceous fuel, was passed through a water trap at the reactor temperature to produce a gas stream nearly saturated in water vapor. This stream was then passed through the reactor at a flow of about 1 liter per minute for 3 or 4 hours to complete one full cycle. At the end of this time, the heating step was begun again to start another cycle. The CO collected in the storage tank and the water in the trap were weighed after each cycle to folb. heating the 3Nal-ICO Na,CO thus formed to a temperature of from about 150 to 250 C. to decompose it into CO H and Na CO and c. recovering the CO low the performance ofthe system. 2. The process of claim 1 wherein the amount of water vapor in the gas is sufficient to provide at least 1 mole ofwater A total of 124 cycles were carried out in the above per l fCO descnbed manner. The results of representative cycles are 3 Th process f claim 1 wherein the gas contains from summarized in Table l. In Table l, the point at which the gas ab t 3 t 25 volume percent water vapor. flow was discontinued and the temperature increased can be 4. The process of claim 1 wherein acid gases are removed determmed by a sharp P from the gas before contacting it with the sodium bicarbonate.

Elapsed lllllt Prcs- Gas flow (ml/mm.) Wt. CO? Wt. Hit) (miii- 'lmuporasure, recovered, recovered, \'t'liiiltis) taro, C. p.s.i.g. N- CO2 01 NO gm. gm.

TABLE I. DATA ON CO2 RECOVERY FROM SYNTHETK STACK GAS Elapsed time Prcs- Gas flow (ml./nim.) Wt. CO1 Wt. H9O (min- Tenipcrasum, recovered, recovered, Cycle lites) turn, C. p.s.i.g. N: CO: 02 gm. gm.

Initial (l 55 130 150 160 175 210 5 0 50 90 55 240 55 275 150 345 ll 0 7 240 65 260 157 360 12mm, 0 5t) 45 72 1135 till 240 (is 255 130 78... 0 65 7 125 285 180 S8 .t 0 65 190 (37 205 130 305 180 in)... 0 65 195 6a 210 125 330 1x5 llti. 0- (ill ixri ll? mu 1:10 2x0 177 is 251.0 ll). I

We claim: 5. The process of claim 1 wherein the sodium carbonate has 1. A process for recovering CO from water vapor containing gases produced by the combination of carbonaceous material, said gases containing from 16 to l8 percent CO along with significant amounts of oxygen, nitrogen, and nitric oxide, which comprises:

a. contacting the gases at a temperature within the range of from about 40 to 75 C. with solid porous sodium carbonate granules having a BET surface area of greater than 0.3 m' lgm. thereby forming 3NaHCO Na CO a BET surface area ofO.5 to 2.0 m /gm.

6. The process of claim 1 wherein the sodium carbonate is mixed with a porous inert material.

7. The process of claim 6 wherein the inert material is foam glass.

8. The process of claim 1 wherein the gases and sodium bicarbonate granules are contacted at a temperature of from 50 to 65 C. 

2. The process of claim 1 wherein the amount of water vapor in the gas is sufficient to provide at least 1 mole of water per mole of CO2.
 3. The process of claim 1 wherein the gas contains from about 3 to 25 volume percent water vapor.
 4. The process of claim 1 wherein acid gases are removed from the gas before contacting it with the sodium bicarbonate.
 5. The process of claim 1 wherein the sodium carbonate has a BET surface area of 0.5 to 2.0 m2/gm.
 6. The process of claim 1 wherein the sodium carbonate is mixed with a porous inert material.
 7. The process of claim 6 wherein the inert material is foam glass.
 8. The process of claim 1 wherein the gases and sodium bicarbonate granules are contacted at a temperature of from 50* to 65* C. 